Use rust-cache action in ci

.github/workflows/ci.yml

@@ -14,6 +14,7 @@ jobs:
           toolchain: stable
           override: true
           components: clippy, rustfmt
+      - uses: Swatinem/rust-cache@v2
       - uses: actions-rs/clippy-check@v1
         if: ${{ github.actor != 'dependabot[bot]' }}
         with:
@@ -38,6 +39,7 @@ jobs:
           profile: minimal
           toolchain: stable
           override: true
+      - uses: Swatinem/rust-cache@v2
       - run: cargo install mdsh mandown
       - run: cargo build --release
       - run: ./build-manpages.sh